This invention relates to a lubricating oil composition having particularly improved friction reducing properties. More particularly, this invention is directed to a lubricating oil composition which contains a combination of an ester of polycarboxylic acid and glycol or glycerol with a selected metal dithiocarbamate to provide improved friction reducing properties.
There has been considerable effort in recent years to develop lubricating oil compositions which will reduce friction in engines, and particularly, automotive engines. This effort is based on the need to improve the fuel economy of such engines which operate on petroleum fuel, a product having a declining source of supply and escalating price. It is known that high engine friction causes significant energy loss and thus, one way to improve fuel economy is to reduce such friction.
Known ways to solve the problem of energy losses due to high friction e.g., in crankcase motor oils include the use of synthetic ester base oils which are expensive and the use of insoluble molybdenum sulfides, which have the disadvantage of giving the oil composition a black or hazy appearance.
Various friction reducing additives have been disclosed in the art as well as many other additives known for providing antioxidant, antiwear, corrosion inhibiting and other useful properties. While such known additives may in fact satisfy one or more of these properties, it is also known that many additives act in a different physical or chemical manner and often compete with one another, e.g., they may compete for the surface of moving metal parts which are subjected to lubrication. Accordingly, extreme care must be exercised in the selection of these additives to insure compatibility and effectiveness.
One additive combination found in lubricating oil compositions and providing excellent antifriction and antiwear properties is an ester of a polycarboxylic acid with glycol and zinc dihydrocarbyl dithiophosphate as disclosed in U.S. Pat. No. 4,105,571. A number of oil soluble molybdenum compounds have been disclosed as useful to provide different lubricant oil properties such as antiwear and friction reduction as shown; e.g., in U.S. Pat. Nos. 4,164,473; 4,176,073; 4,176,074; 4,192,757; 4,248,720; 4,201,683 and 4,289,635, as well as Japanese Pat. No. 56000896.
Another particular group of additives which have been widely used in lubricant compositions are the metal dihydrocarbyl dithiophosphates. These compounds are known to exhibit antioxidant and antiwear properties. While such compounds have been quite successful in providing such improved properties in lubricant compositions, they do contain phosphorus which has been known to cause some deterioration problems in certain catalyst containing automotive systems.
Accordingly, there is the need for providing a lubricant oil composition having friction reducing properties and a limited or relatively reduced level of phosphorus content while retaining other desired properties such as viscosity stability.